The design and performance of an instrument to measure cosmic microwave background (CMB) anisotropy on angular scales from 10’to 30’is reported. The instrument utilizes a single‐mode waveguide bolometer cooled by an adiabatic demagnetization refrigerator. This is the first time a 100 mK bolometer has been used in a CMB experiment. Observations of the CMB were made using this radiometer from the South Pole during the 1991–1992 season and, with minor modifications, during the 1992–1993 season. The radiometer is used with a fully steerable altitude‐azimuth Cassegrain telescope built for this observation, also described here.